text: The hapticity of a ligand can change in the course of a reaction. E.g. in a redox reaction:
  Here one of the η-benzene rings changes to a η-benzene.
  Similarly hapticity can change during a substitution reaction:
  Here the η-cyclopentadienyl changes to an η-cyclopentadienyl, giving room on the metal for an extra 2-electron donating ligand L. Removal of one molecule of CO and again donation of two more electrons by the cyclopentadienyl ligand restores the η-cyclopentadienyl. The so-called indenyl effect also describes changes in hapticity in a substitution reaction.